Microfluidic liquid chromatography (LC) columns, or chips, packed with chromatographic particles require frits at an outlet and in most cases at an inlet of the column in order to retain a packed bed and ensure stable operation over time. In conventional LC columns with an inner diameter of 2.1 or 4.6 mm, for example, frits are typically made out of stainless steel or titanium particles sintered together, or screens, meshes and composites, that are mechanically press-fit into the ends of the column. In columns made out of fused silica capillaries, with inner diameters 75 to 150 microns, the frits are typically made with chemical means by dipping the end of the capillary into a polymeric solution that locks the ends of the column in place when it cures. Another method for capillary columns is to burn a section of the capillary to fuse the particles together.
Frits can be made before or after packing. For example, it is possible to frit the outlet of the column and pack against it, then frit the inlet. Alternatively, a temporary external frit can be placed against the outlet of the column during packing, for example in the outlet capillary that carries the slurry liquid to waste, and removed after packing is complete, after which permanent fits are made on the capillary column at the outlet and possibly the inlet.
In planar microfluidic devices for LC applications, there can be several possible methods for fabricating frits. The geometry of such a device is shown schematically in FIG. 1. In this illustration, the device 100 is made by joining two layers. The top layer 101 contains two through-holes, or vias, 102. The lower layer 103 contains a groove 104. After alignment and joining of the two layers, a hermetically sealed channel is formed. Fittings are attached to the device 100 in order to make fluid connections to the vias 102. Particles dispersed in a slurry flow through the inlet via into the channel and out of the outlet via. A frit, either temporary or permanent, must be placed in the outlet via or after the outlet via in order to retain particles. If the frit is temporary and removed after packing is complete, a permanent outlet frit is made after packing. This frit must be capable of withstanding substantial force applied in subsequent operation without moving or rupturing. Frit motion would likely cause degradation of the column performance. Frit rupture would typically cause complete failure of the column. Typically, a permanent inlet frit is also created so that the packed bed is firmly locked into place.
A first class of frits for microfluidic devices uses chemical solutions such as silicate. A drop is placed at the vias and allowed to cure, upon which the polymeric solution creates bridges between the particles and physically locks them in place. Another class of frits for microfluidic devices employs frits that are micro-machined along with other features of the device, such as channels and vias. In prior art techniques, the physical restriction that achieves retention of the particles inside the microfluidic device is an integral component of the device and is fabricated along with it. As such, the retaining device in those situations is fabricated prior to packing the particles into the microfluidic columns. Therefore, those retaining devices can only be fabricated at the column outlet.
Mechanical frits, typically made out of stainless steel or titanium particles compacted and sintered together, are a standard method for creating retaining structures in traditional LC columns. Typically, in a traditional column (see FIG. 2), made from a stainless steel tube 200, a first frit ring assembly 201, consisting of a frit 203 inside a frit ring 202, is placed at the outlet end of the tube 200 and maintained in place by an outlet end nut 204. Particles in a slurry are packed against this frit. A second frit assembly 205 is placed against the inlet of the tube 200 after packing and maintained in place using the inlet end nut 206.